1. Field of the Invention
This invention relates to a radio communication system which can be used as a mobile telephone system and other similar systems. This invention also relates to a mobile radio communication apparatus used in a radio communication system.
2. Description of the Prior Art
In a general mobile telephone system, the output power of a mobile station is controlled to prevent the interference with other mobile stations.
In the United States, there are known cellular mobile telephone equipments which have a design based on the specification of "Advanced Mobile Phone Service" (AMPS). In such a mobile phone system, the output power of a mobile station is controlled in compliance with a command from a base station to prevent the interference with other mobile stations. In addition, the mobile station controls its output power in accordance with the level of a transmission audio signal to save the power. Saving power is referred to as battery saving in the case of a mobile station of the transportable type of the handheld type which uses a battery (a cell) as a power source.
In such an automobile telephone system, the output power of a mobile station is changed among at most eight different levels in compliance with a command from a base station. The maximum output power level of the mobile station is determined by the power class of the mobile station. The eight different levels are predetermined reference levels separated by intervals of 4 dB. The mobile station is given a tolerance of the output power level which agrees with the range of +2 to -4 dB relative to each of the reference levels. When the base station issues special permission, the mobile station is allowed to decrease its output power up to -8 dB below the designated level. In this case, since the mobile station decreases its output power below the tolerance of the output power level, the mobile station suspends the transmission of a speech supervisory signal (a speech monitor signal) to inform the base station that there is no transmit-audio signal and that the output power is decreased. The speech supervisory signal is composed of a tone signal whose frequency resides outside an audio frequency band, and the tone signal is called a supervisory audio tone in the specification of AMPS. While the base station does not receive any speech supervisory signal from the mobile station, the base station mutes an audio signal transmitted from the mobile station to the other party. While the base station receives a speech supervisory signal, the base station unmutes the audio signal.
As will be explained later, in such a prior-art automobile telephone system, the cancellation of the muting of an audio signal in a base station is liable to delay so that a head of speech tends to fail in the transmission from a mobile station to the other party via the base station.